Inertial position target measuring systems and methods

ABSTRACT

Systems and methods for contiguously and accurately updating target object information during an entire target engagement period are provided. The target tracking system includes a database for storing starfield information, an optical beam source configured to illuminate one or more optical beam pulses, first and second camera systems, and a processor. The processor instructs the first camera system to track the object based on recordation of the tracked object, instructs the second camera system to stabilize the tracking image based on the instructions sent to the first camera system, and determines inertial reference information of the tracked object based on the stabilized image and starfield information associated with the stabilized image.

FIELD OF THE INVENTION

This invention relates generally to target tracking and, morespecifically, to systems and methods for accurately identifying theinertial position of a target.

BACKGROUND OF THE INVENTION

When line of sight tracking is used to determine the inertial positionof objects, accuracy and engagement, timelines may be difficult toachieve. In one approach, an inertial reference unit (IRU) starcalibration update is performed before and after the engagement sequencethereby requiring a longer engagement timeline. When star calibration isperformed before and after the engagement is performed, the IRUinformation diverges between the calibrations, thereby, resulting indegraded inaccurate target object position accuracy.

One process to generate more accurate information of the target objectwould be to temporarily suspend tracking of the target object during thetime of engagement. While tracking is suspended, an IRU star calibrationupdate is performed. However, suspension of tracking during anengagement period may be problematic. For example, if the target objectchanges course during the suspension, then reacquiring the target objectmay become difficult.

However, there exists an unmet need to provide contiguous, accuratetarget object information during an entire target engagement period.

SUMMARY OF THE INVENTION

The present invention provides systems and methods for contiguously andaccurately updating target object information during an entire targetengagement period. The accurate target object information is used toinstruct a weapon or defense system about the target.

In an embodiment of the present invention, an exemplary target trackingsystem includes a database for storing starfield information, an opticalbeam source configured to illuminate one or more optical beam pulses,first and second camera systems, and a processor. The processorinstructs the first camera system to track the object based onrecordation of the tracked object, instructs the second camera system tostabilize the tracking image based on the instructions sent to the firstcamera system, and determines inertial reference information of thetracked object based on the stabilized image and starfield informationassociated with the stabilized image.

In one aspect of the invention, one or more platform information sourcesmay be coupled to the target tracking system to send platforminformation to the target tracking system for use in determininginertial reference information of the tracked object.

In another aspect of the invention, the system may be hosted on aplatform that may include satellite, an aircraft, or a ground basedsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred and alternative embodiments of the present invention aredescribed in detail below with reference to the following drawings.

FIG. 1 is a block diagram of an exemplary target tracking system formedin accordance with an embodiment of the present invention;

FIG. 2 illustrates an exemplary process performed by the system of FIG.1;

FIG. 3 illustrates a geometric representation of an analysis performedby the system of FIG. 1;

FIG. 4 illustrates an exemplary stabilized image generated by acomponent of the system of FIG. 1 for analyzing inertial referenceinformation; and

FIG. 5 is a time graph of a modulated optical beam used in the system ofFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a target tracking system 22 included within aplatform 20, such as without limitation a ground based facility, anaircraft, or a satellite, provides near continuous updating of inertialreference unit (IRU) information while performing uninterrupted opticaltarget tracking. In addition, the platform 20 includes platforminformation sources 26, such as a flight data computer, and an InertialReference System (IRS) that are coupled to the target tracking system22.

In one embodiment, the target tracking system 22 includes a trackingprocessor 30, a high signal light source component 32, a first camera34, a second camera 36, a first fast steering mirror (FSM) 38, a secondFSM 40, a beam splitter 42, and various reflecting mirrors 44. Thetracking processor 30 is operatively coupled to the high signal lightsource component 32, the first and second cameras 34 and 36, the firstand second FSMs 38 and 40, and a database 46. The database 46 storesstarfield reference information for use by the processor 30.

The tracking processor 30 includes an inertial reference unit (IRU) 50.The IRU 50 determines and adjusts inertial reference informationreceived from the IRS based on an optical output of an optical beamsource 58 of the high signal light source component 32 and an imagereceived by the second camera 36. In addition, the tracking processor 30includes a target tracking component 54 that tracks a target displayedwithin an image generated by the first camera 34 and determines inertialreference information of the tracked target based on the inertialreference information determined by the IRU 50 and any information fromthe sources 26. The target tracking component 54 generates aninstruction to the second FSM 40 for stabilizing the tracked image,thereby allowing the second camera 36 to record a stabilized image ofthe starfield. The IRU 50 may be located remote from the processor 30 orthe target tracking system 22.

Referring now to FIG. 2, a process 100 performed by the target trackingsystem 22 (FIG. 1) provides nearly continuous inertial referenceinformation updating using starfield information stored in the database46 without loss of contact of an optically tracked target. The process100 begins after the target tracking system 22 has acquired a target inits view. In other words, the first camera 34 has acquired a targetwithin its field of view and the target tracking component 54 hasanalyzed images generated by the first camera 34 and instructed thefirst FSM 38 to track the identified target. The process 100 begins at ablock 102 where the target tracking component 54 determines the inertialangular rate of the target scan. The target scan inertial angular rateis the speed at which the first FSM 38 moves in order to track thetarget. At a block 106, the starfield image received by the secondcamera 36 is stabilized based on the determined target scan rate. Thedetermined target scan rate is sent to the second FSM 40 for de-scanningthe starfield image received from the first FSM 38.

Referring back to FIG. 2, at a block 108 the second camera 36 recordsthe stabilized image over a predetermined period of time. At a block112, during the period of time that the second camera 36 records thestarfield image, the IRU optical beam generator 58 generates an opticalbeam that is pulsed over a finite period of time. At a block 114, thetracking processor 30 or components thereof identifies the locationwithin the stabilized image of when the IRU optical beam was turned onand off during each optical beam pulse. Referring to FIG. 3, time t₁,identifies the location within a stabilized image 300 where an opticalbeam pulse was initiated and time t₂ is the location within the image300 that identifies when the IRU optical beam pulse was turned off.Referring back to FIG. 2, at a block 118, centroids of each optical beampulse are determined based on respective times t₁ and t₂.

At a block 120, the processor 30 compares the centroids to one or morestars located within the stabilized image based on starfield informationstored in the database 46 and adjusts inertial reference informationreceived from the IRU 50 based on the comparison. At a block 122, theprocessor 30 determines inertial reference information for the targetbased on present target tracking information produced by the targettracking component 54, the adjusted inertial reference information, andany information relating to the platform 20, such as without limitationGPS location information, pitch, roll, yaw, or other orientationinformation received from the other sources 26. Platform information mayinclude position, velocity, and attitude from separate inertialnavigation system for transforming target position into a platformbody-fixed coordinate system.

In one embodiment, the optical beam direction is referenced to thetarget based on target tracking information generated by the targettracking component 54 and sent to the light source component 32. Becausethe optical beam is referenced to the tracked target, the pulses trackacross a stabilized image.

Referring now to FIG. 3, a stabilized image 300 includes a plurality ofstars 302 that are identified by a starfield analysis component includedwithin the tracking processor 30. The stabilized image 300 includes aplurality of optical beam pulses recorded by the second camera 36.Centroids 306 of each optical beam pulse 304 are identified based onidentified t₁, and t₂ of the respective pulse 304. The centroids 306 aresimply the center location of each pulse 304. The processor 30determines the location of the centroids 306 relative to the starfieldpattern 302 within the stabilized image 300 in order to generate highlyaccurate inertial reference update information of the target.

FIG. 4 illustrates an exemplary de-scanned (stabilized) image that isrecorded by the second camera 36. Satellite 1 is a tracked target.

The resulting series of pulses from the beam combined with the knowledgeof the precise time of each pulse allows accurate measurement of theoptical beam pointing direction relative to the starfield. Thismeasurement is input to a Kalman Filter which estimates the IRU errors,thereby allowing accurate reporting of the track object position ininertial frame coordinates. The Kalman Filter algorithm is a standard,recognized estimation algorithm for estimating IRU errors. The KalmanFilter would be applied in the same manner as if the track wereinterrupted and the star measurements taken independent of the trackprocess. The errors in the inertial system which would be estimated arethe three components of inertial attitude, the three components of gyrobias, and the 3 components of gyro scalefactor. The Kalman Filter wouldtherefore be at least a 9 state estimation algorithm. The equations forthe Kalman Filter are given in the literature but are shown here forcompleteness: $\begin{matrix}{K_{n} = {P_{n}{H_{n}^{T}( {{H_{n}P_{n}H_{n}^{T}} + R} )}^{- 1}}} & {{Kalman}\mspace{14mu}{Gain}} \\{x_{n + 1} = {x_{n} + {K_{n}( {z_{n} - {H_{n}x_{n}}} )}}} & {{State}\mspace{14mu}{Update}} \\{P_{n + 1} = {P_{n} - {K_{n}H_{n}P_{n}}}} & {{Co}\mspace{14mu}{variance}\mspace{14mu}{Update}}\end{matrix}$ $\begin{matrix}{{K_{n} = {9 \times 9\mspace{14mu}{Initial}\mspace{14mu}{Co}\mspace{14mu}{variance}\mspace{14mu}{Matrix}\mspace{14mu}({Identity})}},} \\{{H_{n} = {2 \times 9\mspace{14mu}{Measurement}\mspace{14mu}{Matrix}}},} \\{{P_{0} = {9 \times 9\mspace{14mu}{Initial}\mspace{14mu}{Co}\mspace{14mu}{variance}\mspace{14mu}{Matrix}\mspace{14mu}({Identity})}},} \\{R = {2 \times 2\mspace{14mu}{Measurement}\mspace{14mu}{Noise}\mspace{14mu}{Matrix}}} \\{x_{o} = {9 \times 1\mspace{14mu}{State}\mspace{14mu}{Vector}\mspace{14mu}( {{Zero}\mspace{14mu}{Vector}} )}} \\{{z_{n} = {2 \times 1\mspace{14mu}{Measurement}\mspace{14mu}{Vector}}}\mspace{31mu}} \\{( {{Inertial}\mspace{14mu}{Angles}\mspace{14mu}{to}\mspace{14mu}{Each}\mspace{14mu}{Star}\mspace{14mu}{Observed}} )}\end{matrix}$

The Kalman Filter equations are iterated over each exposure time of thestreak camera 36 with a star measurement comprising the Z_(n) vector ateach exposure time.

The optical beam has a signal-to-noise ratio of approximately 100. Thebeam can be very intense relative to the signal returned from thetarget, thereby allowing centroid measurement of the optical beamstreaks to approximately 1/100 of the camera 36 pixel angular extent.

Referring to FIG. 5, a time graph 350 illustrates example on/off timesof an optical beam. The following are exemplary system values associatedwith the graph 350:

-   -   Aperture size: 50 cm (mirror #44)    -   Optical Magnification=10_(x) 1(mirror #44)    -   Slew Rate (ω): 1 degree/second 917.4 mrad/sec) (system #22)    -   Second camera 36        -   1 degree field-of-view        -   4096×4096 array        -   4.25 μrads IFOV    -   Optical beam pulse repetition frequency: 300 hz    -   Angular length of each streak (pulse): 10 pixels (42.5 μrads)    -   Time length of each pulse: 42.5×10⁻⁶/17.4×10⁻³=2.4 msec    -   De-scan time=7.8 msec (assuming two complete pulses in scan        time)    -   Required De-scan FSM 40 angle range=7.8×10⁻³×17.4×10⁻³˜136        μgrads (output space)

While the preferred embodiment of the invention has been illustrated anddescribed, as noted above, many changes can be made without departingfrom the spirit and scope of the invention. Also, the steps in theprocess 100 may be performed in various order. Accordingly, the scope ofthe invention is not limited by the disclosure of the preferredembodiment. Instead, the invention should be determined entirely byreference to the claims that follow.

1. A targeting platform comprising: a target tracking system including:a database for storing starfield information; an optical beam sourceconfigured to illuminate one or more optical beam pulses; a first camerasystem configured to track an object and record the tracked object; asecond camera system configured to stabilize a tracking image receivedby a portion of the first camera system and record the stabilized imagethat includes the one or more optical beam pulses; and a processorcoupled to the database, the optical beam source, and the first andsecond camera systems, the processor including: a first componentconfigured to instruct the first camera system to track the object basedon recordation of the tracked object; a second component configured toinstruct the second camera system to stabilize the tracking image basedon the instructions sent to the first camera system; and a thirdcomponent configured to determine inertial reference information of thetracked object based on the stabilized image and starfield informationassociated with the stabilized image.
 2. The platform of claim 1,further comprising: one or more platform information sources coupled tothe target tracking system, the one or more platform information sourcesbeing configured to send platform information to the target trackingsystem, wherein the third component is configured to further determineinertial reference information based on platform information.
 3. Theplatform of claim 1, wherein the first camera system includes: a firstfast steering mirror configured to track the object based oninstructions from the first component; and a first camera configured torecord an image reflected by the fast steering mirror and send therecorded image to the first component.
 4. The platform of claim 3,wherein the second camera system includes: a second fast steering mirrorconfigured to stabilize the image reflected by the first fast steeringmirror based on instructions sent from the second component; and asecond camera configured to record an image reflected by the second faststeering mirror and send the recorded image to the third component. 5.The platform of claim 1, wherein the platform is a satellite.
 6. Theplatform of claim 1, wherein the platform is an aircraft.
 7. Theplatform of claim 1, wherein the platform is a ground based system. 8.The platform of claim 7, wherein the ground based system is a vehicle.9. A target tracking system comprising: a database for storing starfieldinformation; an optical beam source configured to illuminate one or moreoptical beam pulses; a first camera system configured to track an objectand record the tracked object; a second camera system configured tostabilize a tracking image received by a portion of the first camerasystem and record the stabilized image that includes the one or moreoptical beam pulses; and a processor coupled to the database, theoptical beam source, and the first and second camera systems, theprocessor including: a first component configured to instruct the firstcamera system to track the object based on recordation of the trackedobject; a second component configured to instruct the second camerasystem to stabilize the tracking image based on the instructions sent tothe first camera system; and a third component configured to determineinertial reference information of the tracked object based on thestabilized image and starfield information associated with thestabilized image.
 10. The system of claim 9, wherein the third componentis configured to further determine inertial reference information basedon received platform information.
 11. The system of claim 9, wherein thefirst camera system includes: a first fast steering mirror configured totrack the object based on instructions from the first component; and afirst camera configured to record an image reflected by the faststeering mirror and send the recorded image to the first component. 12.The system of claim 11, wherein the second camera system includes: asecond fast steering mirror configured to stabilize the image reflectedby the first fast steering mirror based on instructions sent from thesecond component; and a second camera configured to record an imagereflected by the second fast steering mirror and send the recorded imageto the third component.